1. Field of the Invention
This invention relates to valves intended to vary or modulate the flow of gaseous fuel to a burner in response to a change in load.
2. Discussion of the Background
It is often desirable in the design of gas-fired equipment to provide a gas fuel delivery apparatus that can automatically vary the flow of gas to the burner in response to a change in load. Such a system is appropriate in many applications, including circulating boilers, water heaters, cooking equipment, gas fireplaces, radiant heaters, and forced-air furnaces. Regarding an instantaneous water heater, for example, water flows through the heat exchanger at variable rates depending on the hot water withdrawal rate at one or more remote taps. In addition to variable flow rate, the water may enter the heater at varying temperatures depending, for instance, on the season of the year. Since the intent of the heater is to deliver hot water at a specified temperature, it follows that the burner must deliver heat at a rate proportional to the flowrate through the heat exchanger and the temperature rise from inlet to outlet that accords with the desired outlet temperature. Many instantaneous water heaters incorporate a mechanism which varies the gas flowrate to the burner in response to changes in the load placed on the heater as described above.
A similar situation with regard to varying loads can pertain to hot water circulating boilers as well. In this case, the boiler is part of a circuit through which water or some other fluid is pumped. In some instances, the flowrate through the boiler can vary; for instance in a zone heating circuit served by one or more pumps. Also, a change in load can be reflected in a change in the temperature rise effected in the water passing through the boiler. In some boiler applications, it is desirable to run the boiler at various outlet temperatures, depending for instance on the outdoor temperature (space heating application) or domestic hot water draw (for the case where the boiler also heats domestic water either directly or indirectly through another heat exchanger).
For certain types of cooking equipment, it is desirable to maintain proper cooking temperature regardless of the load on the appliance. In a conveyor oven, for example, food to be cooked may be heavily or lightly loaded onto the conveyor. It is appropriate to apply automatic gas input modulation to such an appliance to maintain the proper cooking temperature at all loads without manual intervention.
Automatic modulating gas valves of different types have been used with gas-fired equipment. It is important to distinguish between automatic valves which modulate continuously over a range of inputs and automatic valves which snap from a high input to a low input when the setpoint temperature is reached, then snap back to the high input when the temperature drops a certain amount below the setpoint. Typical of valves in the latter category are the automatic input control valves used for residential gas ovens. These valves, while adequate for their application, are not modulating valves in the true sense. Further discussion herein of automatic modulating gas valves pertains only to valves of the former type, that is automatic gas valves which modulate continuously over a prescribed range of input. Other than the present invention, the three types which provide for automatic input modulation over a prescribed range of input are:
1. Immersion bulb modulating valve. This is the most common type of modulating gas valve. It is a diaphragm-actuated valve in which the diaphragm is pressurized by a fluid-filled bulb connected to the valve with a capillary tube. The bulb is immersed in the discharge of the heating appliance (generally a furnace or boiler), thereby causing the valve to modulate the gas flow in seeking the setpoint temperature. A manual adjustment of setpoint temperature is usually provided as a knob either on the valve itself or in the capillary line between the bulb and the valve. The limitations of this approach are PA1 2. Electronically-actuated variable regulator. The valve uses an electric coil actuator to vary the force applied to a diaphragm. There is also a mechanical means by which the minimum flow through the valve can be set. Otherwise, the valve is constructed similar to a normal gas regulator. The valve is used with an electronic controller which provides a variable current to the coil actuator. The variable current results in a correspondingly variable force against the diaphragm. PA1 3. The third category of automatic modulating valves is a special purpose valve used on some instantaneous water heaters. This valve utilizes an immersed bulb and capillary tube, and also includes a mechanism which causes the valve to respond to a change in water flowrate through the appliance.
1. Setpoint adjustment is manual, imprecise, and must be located in physical proximity to the valve. PA2 2. The range of setpoints is limited, normally to 120.degree. F. or less. Typical is a range of 60.degree.-100.degree. F. Applications in commercial cooking appliances usually require capabilities for higher set points. PA2 3. In a system involving multiple setpoints, this approach cannot be used without a cumbersome gas train design comprising multiple parallel feed lines and control valves. This is pertinent to current commercial air heaters incorporating modulating control and developments now occuring with modulating furnaces and boilers.
These three types of automatic modulating gas valves effect a variation in gas flow by modulating the pressure of the gas in the manifold to which is attached one or more fixed orifices which discharge into the mixing tube or tubes of a gas burner. Such a burner is normally a Bunsen-type atmospheric burner, but it may also be an induced-draft power burner. In the latter system, the pressure in the combustion chamber is subatmospheric due to an induced draft blower in the fluegas discharge vent. In such a system, all or most of the combustion air, along with the fuel gas, is drawn through the mixing tube of the burner. Variation of the manifold pressure may be called the first modulating method pertaining to a burner which incorporates a mixer tube. The second modulating method is to utilize a valve which effects a variable orifice through which gas is discharged directly into the mixer tube of the burner. The second method is to vary the area of the discharge orifice while keeping the pressure drop across it constant. Thus, the variation in gas flow is effected by a variable orifice area with a constant pressure drop rather than by a constant orifice area with a variable pressure drop.
Whether the first or second method of modulation is appropriate depends on the application. For a burner which is fed by multiple mixer tubes, the first method is appropriate, since one modulating valve can act to vary the pressure throughout the gas manifold. For an atmospheric burner with a single mixer tube, the second method can offer significant performance advantages over the first method (a detailed discussion of this issue may be found in the parent patent application). For an induced-draft power burner, either method may be used without a difference in operating characteristics, since the induced draft design gives the same fuel/air ratio regardless of how the gas jet is introduced into the mixer tube.